Hydraulic load matching device

ABSTRACT

The invention consists of a device for use in an artificial heart which efficiently matches the force/displacement characteristics of a thermodynamic engine to the force/displacement characteristics of the blood. The matching is accomplished hydraulically and allows for interruption of the engine cycle periodically for control purposes.

United States Patent Hanson 1 May 27, 1975 [54] HYDRAULIC LOAD MATCHINGDEVICE 3,080,820 3/1963 Browne 417/383 3,257,8[0 6 1966 H 60 547 [75]Inventor: John P. Hanson, McKeesport, Pa. mm [73] Assignee: The UnitedStates of America as FOREIGN PATENTS 0R APPLICATIONS represented heUnited Sates Franc: A t r r t i .4 Energy Research and DevelopmentAdministration, Washington, DC. Primary Examiner-Martin P. Schwadron[22] Filed: Sem 7 1973 Assistant Examiner-H. Burks, Sr.

Appl. No: 395,219

Attorney. Agent, or Firm-Dean E. Carlson; Robert J. Marchick [S2] U.S.Cl .3 60/583; 4l7/383 [57] ABSTRACT [51] Int. Cl. ..F15b 7/00 Th t f d f581 Field of Search 3. 60/537, 547, 583, 593, e F 3 1a] heart whichefficiently matches the forceldisplace- 60/533, 585, 417/383 mentcharactensncs of a thermodynamic engine to the [56] Rererences Citedforce/displacement characteristics of the blood. The matchlng isaccomplished hydrauhcally and allows for UNITED STATES PATENTSinterruption of the engine cycle periodically for con- 777,008 l2/l904Defies et al 60/593 [r0] purposes. 2,490,118 l2/l949 Dickinson 4l7/3833,044,267 7/1962 Hicks 60/533 3 Claims, 2 Drawing Figures v 20 12 15 171s 4 l 14 as F f 2 22 r V 11 '7 i 25 PATENTEDMAY 2 7 I975 SHEET N MI:228% ust m5 8 Q0 3 to 8 N6 3 o I I o SEZE MSBG m o5 5 T BEmS mxomhm 4 5mmo .zmom um HYDRAULIC LOAD MATCHING DEVICE BACKGROUND OF THE INVENTIONDESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, athermodynamic engine cylin- The imention relates to a load matchingdevice use' 5 der ll contains a piston I2 which drives a piston I3,

ful in a heart pump which may be implanted in a human body forreplacement of the natural heart. The need in artificial hearts is for asimple, compact, lightweight, reliable, low energy requirement pumphaving the capability of closely approximating the pumping ability ofthe normal human heart. The pump must correspond functionally as nearlyas possible with the natural heart.

The load matching device of the present invention is designed to couplethe output of a thermodynamic engine to a load such as a blood pump. Forbest efficiency, the working fluid of a thermodynamic engine must beallowed to expand during a portion of the power stroke. As this occurs,the pressure within the cylinder drops. For this reason, the forcedeveloped by the piston varies through the power stroke. For mostapplications, the load does not match this force variation, and somemeans must be found of storing the en ergy during the first portion ofthe stroke and releasing it during the later low pressure portions ofthe stroke. For most application, where the speed is not expected tovary significantly from stroke to stroke, this is accomplished byconverting the reciprocating motion of the piston to rotary motion, thusallowing the excess energy to be stored as a small increase in theangular velocity of a flywheel. However, there are applications wherethe engine must come to a complete stop at the end of each stroke, andthe use ofa flywheel to match the engine characteristics to the loadcharacteristics would not be convenient.

One exampie of an application where the use of a flywheel for forcematching between the prime mover and the load is inconvenient is the socalled heart rate artificial heart. In such a device, the blood pump isallowed to fill at a rate determined by the venous blood pressure. Whenthe blood pump fills completely, the engine is triggered, emptying theaccumulated blood into the aorta. As the pump discharge pressure isessentially constant over the power discharge stroke, a mis-match existsbetween the prime mover force/displacement characteristics and theforce/displacement characteristics of the blood.

SUMMARY OF THE INVENTION Accordingly. it is an object of the presentinvention to provide an improved fluid flow apparatus which smooths avarying fluid stream.

It is a further object ofthe invention to provide an apparatus forhydraulically matching the force/displacement characteristics of theblood. without the use of valves or accumulators, and without the needfor rotating masses or bearings. The device consists of a liquid filledtube joining a master and slave piston and cylinder.

The invention will be best understood with reference to the followingspecification taken in conjunction with the accompanying drawings, ofwhich:

FIG. 1 is a view showing the structure of the hydraulic load matchingdevice.

FIG. 2 shows a typical displacement/time curve for hydraulic loadmatching device as applied to a blood pump.

the pistons being connected by a shaft 14. A return spring 15 is locatedwithin engine cylinder II around shaft [4 in order to improve pistonreturn timing. Piston I3 is enclosed within an master cylinder 16, whichis the first stage in the load matching device of the present invention.

In operation, the force transmitted from the engine to the mastercylinder I6 forces fluid through a nozzle 17 and then into atransmission tube [8. Part of the input force is transmittedhydrostatically through a diffuser 19 to slave cylinder 20, causing thepiston 2]. contained within the slave cylinder 20, to move linearly. Anyexcess force provided at the master cylinder appears as a positivepressure difference between the master and slave cylinder. The pressuredifference accelerates the liquid in the transmission tube. providingenergy storage in the liquid in the form of kinetic energy. When theforce on the master cylinder falls below that needed to move the load ata steady rate, the liquid decelerates. converting the kinetic energy ofthe moving fluid into a negative pressure difference, and allow ing theload to keep moving.

FIG. 2 presents a typical displacement/time curve for a system using theload matching device of the subject invention. The timing has beenarbitrarily set at 0.2 seconds for the systole (pumping) part of thestroke and 0.3 seconds for the diastole (return) part ofthe stroke.

In operation, the load could be a blood pump 22. In this case, the inletvalve 23 of the pump is closed while the output valve 24 is open and thepressure in the slave cylinder 20 is proportional to the arterialpressure during systole. This pressure varies from approximately mm Hgto approximately I20 mm Hg during the stroke. The actual pressure woulddepend on the area ratio between the slave cylinder piston 21 and thatof the blood pump 22. During diastole, the pump body is connected to theveins, and the pressure is essentially zero. The pump body could befitted with a flexible dome 25 to prevent the return spring fromcollapsing the veins during diastole.

The actual size of the transmission tube 18 will depend on the arearatio between the slave cylinder 20 and the pump diaphragm, as well asthe working fluid selected. In addition, the engine expansion ratio andthe desired timing will affect the dimensions of the device. Using thetiming shown in FIG. 2, with a lzl area ratio between slave cylinder andpump diaphragm, the displacement volume should be approximately cc toprovide 12 liters/minute of blood flow. Using water as the transmissionfluid, the length of the tube should be about 3 feet with a diameter ofabout inch in order to obtain the performance shown in FIG. 2. This isperhaps longer than needed to connect a typical engine and pump, but theexcess length could be coiled around the engine housing with little lossin performance. The dimensions of the master and slave cylinders can bevaried to suit the installation as long as the displaced volume is keptconstant.

What is claimed is:

l. A device for matching load characteristics to engine characteristicscomprising:

an input shaft from an associated engine cylinder;

first piston at the end of said shaft;

master cylinder containing said shaft and said piston:

nozzle at one end of said master cylinder; cylindrical transmission tubeat the converging end of said nozzle;

conical diffuser at the opposite end of said transmission tube;

slave cylinder connected to said diffuser at the divergent end of saiddiffuser;

second piston contained within said slave cylinder;

1. A device for matching load characteristics to engine characteristicscomprising: an input shaft from an associated engine cylinder; a firstpiston at the end of said shaft; a master cylinder containing said shaftand said piston; a nozzle at one end of said master cylinder; acylindrical transmission tube at the converging end of said nozzle; aconical diffuser at the opposite end of said transmission tube; a slavecylinder connected to said diffuser at the divergent end of saiddiffuser; a second piston contaiNed within said slave cylinder; and anincompressible fluid contained within said master cylinder, said nozzle,said transmission tube, said diffuser, and said slave cylinder fortransmission of motion between said first piston and second piston. 2.The device of claim 1 wherein said transmission tube is of a diameter of1/2 inch.
 3. The device of claim 2 wherein said incompressible fluid iswater.